Thermocompression bonding is a technique for joining two articles by simultaneously applying heat and pressure to an interface between the articles. Typically, neither the heat nor the pressure alone is sufficient to produce the desired bond. However, when both heat and pressure are appropriately combined a bond is produced at the interface.
Thermocompression bonding is frequently used in electronic manufacturing processes for joining electrical leads to circuits formed on substrates, particularly ceramic substrates. Typically, bond pads of the same type surface metallization as that of the leads are located along edges of the substrates. All leads along a particular edge of a substrate are typically aligned with, and simultaneously urged against, their respective pads by a bonding rail of a bonding head.
Since a predetermined, controllable amount of both pressure and heat are necessary for forming the bond so as not to damage the substrate or circuit by an excess of either, it has always been an endeavor of the practitioner to distribute the heat and pressure equally among all leads to be bonded during the same operation to obtain bonds of equal strength between all of the leads and the substrate. Heat and pressure are transferred to the leads through a major element of the bonding head which is frequently referred to as a thermode. The thermode is typically a metal body which houses one or more heating elements. A surface of the body of the thermode is typically shaped into one or more bonding rails.
Such a bonding rail bears upon any predetermined number of leads and their respective pads as are to be bonded along an edge of a substrate. In contacting the leads, the rail transfers a compressive force as well as the heat from the thermode to the leads. The rail is, consequently, subjected to severe compressive and thermal cycling stresses, as it cyclically engages and disengages the leads during successive bonding operations.
A distribution of compressive forces among all leads along the length of the rail requires delicate adjustments of the thermode with respect to a mounting frame of the head. The frame is typically mounted in a bonding apparatus to slide or translate toward and away from a bonding platform. The adjustments of the thermode orient the rail in parallel to the plane of a bonding platform which supports the substrate and the leads.
According to the prior art such adjustments are made each time a bonding rail has become deformed to a degree at which the uniformity and quality of bonds made by the rail is no longer acceptable.
Such adjustments of the thermode are typically made by trial and error. After each adjustment a group of leads are bonded to a substrate and the bonded leads are checked for uniformity of deformation in the leads. The adjustments of the thermode with respect to the frame are repeated until the deformation among the bonded leads has become uniform. A significant period of time is consequently devoted to adjusting the thermode.